1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
|
<!-- doc/src/sgml/pgwalinspect.sgml -->
<sect1 id="pgwalinspect" xreflabel="pg_walinspect">
<title>pg_walinspect</title>
<indexterm zone="pgwalinspect">
<primary>pg_walinspect</primary>
</indexterm>
<para>
The <filename>pg_walinspect</filename> module provides SQL functions that
allow you to inspect the contents of write-ahead log of
a running <productname>PostgreSQL</productname> database cluster at a low
level, which is useful for debugging, analytical, reporting or
educational purposes. It is similar to <xref linkend="pgwaldump"/>, but
accessible through SQL rather than a separate utility.
</para>
<para>
All the functions of this module will provide the WAL information using the
current server's timeline ID.
</para>
<para>
All the functions of this module will try to find the first valid WAL record
that is at or after the given <replaceable>in_lsn</replaceable> or
<replaceable>start_lsn</replaceable> and will emit error if no such record
is available. Similarly, the <replaceable>end_lsn</replaceable> must be
available, and if it falls in the middle of a record, the entire record must
be available.
</para>
<note>
<para>
Some functions, such as <function><link
linkend="pg-logical-emit-message">pg_logical_emit_message</link></function>,
return the LSN <emphasis>after</emphasis> the record just
inserted. Therefore, if you pass that LSN as
<replaceable>in_lsn</replaceable> or <replaceable>start_lsn</replaceable>
to one of these functions, it will return the <emphasis>next</emphasis>
record.
</para>
</note>
<para>
By default, use of these functions is restricted to superusers and members of
the <literal>pg_read_server_files</literal> role. Access may be granted by
superusers to others using <command>GRANT</command>.
</para>
<sect2>
<title>General Functions</title>
<variablelist>
<varlistentry>
<term>
<function>pg_get_wal_record_info(in_lsn pg_lsn) returns record</function>
</term>
<listitem>
<para>
Gets WAL record information of a given LSN. If the given LSN isn't
at the start of a WAL record, it gives the information of the next
available valid WAL record; or an error if no such record is found.
For example, usage of the function is as
follows:
<screen>
postgres=# SELECT * FROM pg_get_wal_record_info('0/1E826E98');
-[ RECORD 1 ]----+----------------------------------------------------
start_lsn | 0/1E826F20
end_lsn | 0/1E826F60
prev_lsn | 0/1E826C80
xid | 0
resource_manager | Heap2
record_type | PRUNE
record_length | 58
main_data_length | 8
fpi_length | 0
description | snapshotConflictHorizon 33748 nredirected 0 ndead 2
block_ref | blkref #0: rel 1663/5/60221 fork main blk 2
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>
pg_get_wal_records_info(start_lsn pg_lsn, end_lsn pg_lsn)
returns setof record
</function>
</term>
<listitem>
<para>
Gets information of all the valid WAL records between
<replaceable>start_lsn</replaceable> and <replaceable>end_lsn</replaceable>.
Returns one row per WAL record. If <replaceable>start_lsn</replaceable>
or <replaceable>end_lsn</replaceable> are not yet available, the
function will raise an error. For example:
<screen>
postgres=# SELECT * FROM pg_get_wal_records_info('0/1E913618', '0/1E913740') LIMIT 1;
-[ RECORD 1 ]----+--------------------------------------------------------------
start_lsn | 0/1E913618
end_lsn | 0/1E913650
prev_lsn | 0/1E9135A0
xid | 0
resource_manager | Standby
record_type | RUNNING_XACTS
record_length | 50
main_data_length | 24
fpi_length | 0
description | nextXid 33775 latestCompletedXid 33774 oldestRunningXid 33775
block_ref |
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>
pg_get_wal_records_info_till_end_of_wal(start_lsn pg_lsn)
returns setof record
</function>
</term>
<listitem>
<para>
This function is the same as <function>pg_get_wal_records_info()</function>,
except that it gets information of all the valid WAL records from
<replaceable>start_lsn</replaceable> till the end of WAL.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>
pg_get_wal_stats(start_lsn pg_lsn, end_lsn pg_lsn, per_record boolean DEFAULT false)
returns setof record
</function>
</term>
<listitem>
<para>
Gets statistics of all the valid WAL records between
<replaceable>start_lsn</replaceable> and
<replaceable>end_lsn</replaceable>. By default, it returns one row per
<replaceable>resource_manager</replaceable> type. When
<replaceable>per_record</replaceable> is set to <literal>true</literal>,
it returns one row per <replaceable>record_type</replaceable>.
If <replaceable>start_lsn</replaceable>
or <replaceable>end_lsn</replaceable> are not yet available, the
function will raise an error. For example:
<screen>
postgres=# SELECT * FROM pg_get_wal_stats('0/1E847D00', '0/1E84F500')
WHERE count > 0 AND
"resource_manager/record_type" = 'Transaction'
LIMIT 1;
-[ RECORD 1 ]----------------+-------------------
resource_manager/record_type | Transaction
count | 2
count_percentage | 8
record_size | 875
record_size_percentage | 41.23468426013195
fpi_size | 0
fpi_size_percentage | 0
combined_size | 875
combined_size_percentage | 2.8634072910530795
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>
pg_get_wal_stats_till_end_of_wal(start_lsn pg_lsn, per_record boolean DEFAULT false)
returns setof record
</function>
</term>
<listitem>
<para>
This function is the same as <function>pg_get_wal_stats()</function>,
except that it gets statistics of all the valid WAL records from
<replaceable>start_lsn</replaceable> till end of WAL.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title>Author</title>
<para>
Bharath Rupireddy <email>bharath.rupireddyforpostgres@gmail.com</email>
</para>
</sect2>
</sect1>
|
